Solubility Rules for Net Ionic Equations
The following are some ”thumb rules” for deciding whether to use ions or molecules in
writing net ionic equations.
Rule #1 – Binary acids: HCl, HBr, and HI are strong; all other binary acids (and HCN) are
weak. Strong acids are written in ionic form; weak acids are written in molecular form.
Rule #2 – Ternary acids: If the number of oxygen atoms in the inorganic acid molecule
exceeds the number of hydrogen atoms by two or more, the acid is strong. We will consider all
carboxylic acids as weak.
Strong: HClO3, HClO4, H2SO4, HNO3, H2SeO4
Weak: HClO, H3AsO4, H2CO3, H4SiO4, HNO2
Rule #3 – Polyprotic acids: In the second and subsequent ionizations the acids are always
weak, whether or not the original acid is strong or weak.
Rule #4 – Bases: Hydroxides of the Groups 1 and 2 (except beryllium and magnesium) are
strong bases. All others including ammonia, hydroxylamine, and organic bases are weak.
Rule #5 – Salts: Salts are written in ionic form if soluble, and in undissociated from if
insoluble. Use the *Solubility Rules* below.
Ionic: K+ + Cl-, Zn2+ + 2NO3Undissociated: AgBr, BaSO4
Rule #6 – Oxides: Oxides are always written in molecular or undissociated form.
Rule #7 – Gases: Gases are always written in molecular form.
*Solubility Rules*
(For Rule #5 – above)
1. All common salts of the Group 1 elements and ammonium are soluble.
2. All common acetates and nitrates are soluble.
3. All binary compounds of Group 17 elements (other than F) with metals are soluble except
those of silver, mercury(I), and lead.
4. All sulfates are soluble except those of barium, strontium, lead, calcium, silver and
mercury(I).
5. Except those in #1, carbonates, hydroxides, oxides, sulfides, and phosphates are insoluble.